The present invention relates to the treatment of water to inhibit scale and control corrosion of metals in contact with aqueous systems. More particularly, the present invention relates to the use of tetrazolium salts in combination with polyacrylic acid or polymaleic acid to inhibit scale or prevent corrosion of ferrous-based metals in contact with aqueous systems.
In industrial cooling systems, water such as from rivers, lakes, ponds, etc., is employed as the cooling media for heat exchangers. The cooling water from heat exchangers is typically passed through a cooling tower, spray pond or evaporative system prior to discharge or reuse. In these systems, the cooling effect is achieved by evaporating a portion of the water passing through the system. Because of the evaporation which takes place during cooling, dissolved materials in the water become concentrated, making the water more corrosive.
In cooling systems, corrosion causes two basic problems. The first and most obvious is the failure of equipment, resulting in replacement costs and plant downtime. Also, decreased plant efficiency occurs due to the loss of heat transfer. The accumulation of corrosion products causes heat exchanger fouling, resulting in the loss of heat transfer.
Ferrous-based metals, e.g., iron metal and metal alloys containing iron (mild steel), are routinely used in the construction of cooling systems due to their low cost and availability. As the system water passes over or through these ferrous-based metal containing devices, they are subjected to corrosion processes. Corrosion inhibitors are generally added as part of a water treatment program in cooling systems to prevent and inhibit the corrosion of ferrous-based metal containing devices.
Molybdates, zinc, phosphates or polyphosphates, and phosphonates have been used to inhibit the corrosion of ferrous-based metals in contact with the system water of cooling systems. Each treatment, however, presents certain drawbacks.
There exists a need, therefore, for a more environmentally acceptable corrosion inhibitor of ferrous-based metals in contact with aqueous systems.
Preventing the corrosion and scaling of industrial heat transfer equipment is essential to the efficient and economical operation of a cooling water system. Excessive corrosion of metallic surfaces can cause the premature failure of process equipment, necessitating downtime for the replacement or repair of the equipment. Additionally, the buildup of corrosion products on the heat transfer surface reduces efficiency, thereby limiting production or requiring downtime for cleaning.
The present invention provides an effective method and composition for controlling corrosion of metals, particularly ferrous-based metals in contact with aqueous systems.
The method of the present invention comprises treating industrial waters with a tetrazolium salt of the general formula: 
wherein R1, R2 and R3 can be various organic and inorganic substituents, e.g., from the group consisting of lower alkyl, aryl, aralkyl, and heterocyclic substituted aryl with the proviso that neither R1, R2 or R3 contain more than 14 carbon atoms, and n may be 1 or 2, in combination with polyacrylic acid or polymaleic acid.
The tetrazolium compounds may contain positive or negative counter ions in order to balance the charges on the above structure. Chemical or electrochemical reduction of this type of compound produces tetrazolinyls and formazans that readily adsorb on metal surfaces and provide films for corrosion protection.
For example, the tetrazolium compound can be 3,3xe2x80x2-(3,3xe2x80x2-dimethoxy-4,4xe2x80x2-biphenylene)-bis-[2-(p-nitrophenyl)-5-phenyl-2H-tetrazolium chloride] (NBT); 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl tetrazolium chloride; 2,5-diphenyl-3-(1-naphthyl)-2H-tetrazolium chloride; and 2,3,5-triphenyl-2H-tetrazolium chloride.
In aqueous systems, the following corrosion reactions of metals such as steel occur:
Fexe2x86x92Fe2++2exe2x88x92
Fe(OH)2+OHxe2x88x92xe2x86x92Fe(OH)3+exe2x88x92
When tetrazolium compounds possessing redox potentials higher than that of the corroding metals or alloys are employed, reduction of tetrazolium molecules readily occur on the steel surface to form insoluble materials and, hence, prevent steel from further corrosion.